Composites are defined as multiphase materials, which are found in nature or may be man-made. Man-made composites typically are formulated using one or more materials so as to achieve properties that are not available individually. Composites may be classified based on type of continuous matrix and dispersed phases, such as reinforcement. Composite materials wherein one of the constituent phases, primarily the dispersed phase, has at least one dimension on the order of 1-100 nanometers are referred to as “nanocomposites.” Nanocomposites may be further classified based on category (e.g., organic or inorganic), as well as geometry of nanoscale reinforcement. A few well-known examples of naturally occurring nanocomposites include human bone, seashells, spider silk, and armored fish. As will be appreciated, each of these materials comprises a structural hierarchy (structure at multiple length scales) which makes them perform exceptionally well as compared with other materials of a similar chemistry.
Material properties of composites are known to be dependent on interactions between the matrix and the dispersed reinforcement. Large surface areas per unit volume at the nanoscale generally cause nanomaterials to function differently than their bulk counterparts. With increased interactions between the matrix and the dispersed phase, nanocomposites are considered relatively superior to conventional composites, providing advantageously new properties without compromising existing beneficial properties, such as strength or durability.
Polyethylene terephthalate (PET) is an aromatic semi-crystalline thermoplastic polyester, synthesized in the early 1940s. PET is well known for its strength and toughness, high glass transition and melting points, chemical resistance, and optical properties. PET is commonly used for commodity and engineering applications also due to its relatively low cost. PET characterized by a microstructure wherein longitudinal stretching forms strong fibers with high molecular chain orientation, as well as bi-axial stretching forming strong films. Linear PET is naturally semi-crystalline. Thermal and mechanical history, such as rate of cooling and stretching, respectively, can drive PET to be amorphous or more crystalline, and thus influence its mechanical properties. Although PET is utilized in industries such as fiber, packaging, filtration, and thermoforming industries, the use of PET is constrained due to a slow crystallization rate and a limited barrier performance as compared to other polyesters, such as PBT, PTN, and the like.
As will be appreciated, there is a long felt need to develop lightweight materials for use across a range of industries, such as packaging, automotive, and aerospace, thus promoting attempts to improve material properties through better control of material processing and an addition of reinforcements. For example, increasing the crystallinity of PET improves its mechanical and barrier properties. Restrictions with the material, however, such as crystallization rate, and industrial processes in maximizing crystallinity, such as cooling rate, cycle time, and stretching process, have limited attempts to improve the material properties of PET. Progress in the field of nanomaterials, however, has led to a development of PET nanocomposites which have improved the physical properties of PET, thus making PET more effective for applications within the automotive, aerospace, and protective apparel industries. Different types of nanoreinforcements (Clay, CNF, CNT, Graphene, SiO2, etc.) have been found to improve the material properties of PET, such as mechanical, thermal, barrier, electrical, fire retardation, optical, surface properties, crystallization kinetics of PET, and the like.
Exfoliation of nanoreinforcements into individual entities and their uniform dispersion into a polymer matrix is essential for the success of polymer nanocomposites. Uniform dispersion of nanoreinforcements in polymers may be achieved by way of various approaches, including, but not limited to, melt-compounding, in-situ polymerization, surface treatment of the nanoreinforcements, and the like. Carbon nanomaterials, such as carbon nanofibers, carbon nanotubes (CNTs), and graphene generally are advantageous due to their superior material properties and simple chemistry. Multi-fold property improvements can be achieved through the dispersion of carbon nanomaterials into polymers
Graphene is a relatively new nanomaterial which comprises a single layer of carbon atoms similar to an unzipped single walled carbon nanotube. Single layer graphene generally is twice as effective as CNTs in reinforcing polymers since graphene has two surface for polymer interaction whereas a CNT comprises only one exterior surface for polymer interaction. It will be appreciated that a development of graphene synthesis methods in conjunction with an introduction of new graphene-based nanomaterials, such as graphene oxide, expanded graphite, and graphene nanoplatelets, has made graphene commercially viable. However, limited information on the effectiveness of graphene-based nanomaterials has limited their application in fabricating polymer nanocomposites. Thus, there is a need for investigating the influence of graphene nanomaterials in reinforcing polymers.
Melt-compounding and in-situ polymerization have been the most studied techniques for preparing PET-Graphene nanocomposites. Although in-situ polymerization is effective in dispersing graphene, the use of in-situ polymerization is limited due to difficulties in attaining a desired molecular weight and a need for expensive reactors. Melt-compounding is a straight-forward approach involving shear mixing, but that alone has not been found to be effective in dispersing graphene in the several polymer systems tested. As will be appreciated, achieving a homogenous dispersion of the nanoplatelets in PET is critical for improving bulk properties. Dispersing graphene in PET is nontrivial, however, as PET generally is highly viscous (500-1000 Pas) with a melting temperature of 260° C.-280° C. Thus, selecting a process that can allow working at high temperatures and with highly viscous materials is necessary.
Another important aspect for the implementation of polymer nanocomposite applications is an ability to predict their material properties so as to provide flexibility in designing manufacturing processes and to reduce developmental costs. Traditional composite models are not accurate in predicting the properties of nanocomposites. Although micromechanical models based on continuum theory have been found to be effective in estimating short fiber composites, few studies have reported an applicability of these models to nanocomposites.
What is needed, therefore, is an effective process whereby graphene nanoplatelets (GNP) may be uniformly dispersed in PET so as to reinforce bulk PET, and micromechanical models whereby the material properties of reinforced bulk PET may be predicted.
While the present disclosure is subject to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. The invention should be understood to not be limited to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure.